1. Field of the Invention
The present invention relates to an imaging method and an imaging apparatus.
2. Description of the Related Art
In recent years, as an imaging apparatus for ophthalmic application, a scanning laser ophthalmoscope (SLO) and an imaging apparatus employing interference of low coherent light have been developed. The SLO two-dimensionally irradiates a fundus with laser light and receives the reflected light. The imaging apparatus employing the interference of the low coherent light is called an optical coherence tomography (OCT), which is used, in particular, to obtain a tomographic image of the fundus or a vicinity of the fundus. A type of the OCT includes a time domain OCT (TD-OCT) and a spectral domain OCT (SD-OCT), and OCTs using various methods have been being developed.
In particular, in recent years, there has been a progress in achieving an even higher resolution owing to a development of a high numerical aperture (NA) of the irradiation laser in the above-mentioned imaging apparatus for the ophthalmic application. However, when taking an image of a fundus, the image is taken through optical textures of an eye, such as a cornea and a lens. Therefore, as the resolution is increased, the quality of the taken image has become influenced significantly by aberrations of the cornea and the lens.
In order to cope with this problem, researches on an adaptive optics (AO)-SLO and an AO-OCT have been progressed, in which the aberration of an eye is measured and an AO as an adaptive optical system for correcting the aberration is incorporated in an optical system. For example, Document “Y. Zhang et al, Optics Express, Vol. 14, No. 10, 15 May 2006” describes an example of the AO-OCT. In general, the AO-SLO and the AO-OCT measure a wavefront of the eye by using a Shack-Hartmann wavefront sensor system. The Shack-Hartmann wavefront sensor system measures the wavefront of the eye by irradiating the eye with measuring light and receiving the reflected light with a CCD camera through a microlens array. The AO-SLO and the AO-OCT can take a high resolution image by driving a variable shape mirror and a spatial phase modulator to correct the measured wavefront and taking an image of the fundus through the variable shape mirror and the spatial phase modulator.
In an image acquiring apparatus including a general adaptive optical system, a feedback control is performed, in which a process of measuring an aberration of an eye and correcting the aberration based on the measured information is performed repeatedly. The feedback control is performed in order to correct an error generated between an instruction value to a correction device and an actual correction amount, and to respond to fluctuation of an aberration of the eye due to lacrimation or a state of refraction adjustment. Similarly to a general feedback control, the control of aberration correction requires a certain period of time from start of the process until an appropriate aberration correction state is achieved. In particular, because a wavefront sensor or a wavefront correction device used for aberration correction has a low response speed, it takes a few seconds to a few tens of seconds until an appropriate correction state is achieved.
Because a certain period of time is necessary until an appropriate correction state is achieved as described above, a position or state of an object to be inspected changes so that the aberration may be largely changed before an appropriate correction state is achieved, and as a result, the period of time until an appropriate correction state is achieved may be elongated in many cases. For instance, Japanese Patent Application Laid-Open No. 2011-104125 describes a method for reducing the time until an appropriate correction state is achieved by interrupting the control of a correction unit when a position of an eye to be inspected is largely changed.
As described above, there is a case where a state of the eye to be inspected that is not appropriate for imaging may be caused during the feedback control of aberration correction due to a positional change of the eye to be inspected or an influence of eyelash or the like. In this case, because of an influence of the state change, it takes a long time until the aberration is sufficiently reduced, which causes a difficulty in taking an image of the eye to be inspected. In order to take an image of the eye to be inspected, it is preferred that the final aberration be smaller. However, it is possible to take an image thereof when the aberration is corrected to a certain extent. In particular, when the object to be inspected is a human eye, from a viewpoint of mental concentration of a subject or an amount of moisture of the eye, there are many cases where a better result can be obtained by taking an image with a certain extent of correction state rather than by taking a long time to correct sufficiently. However, in the conventional apparatus with an adaptive optical system, it takes a long time until image taking becomes possible, which makes it difficult to take a high quality image of a fundus.